One question, three grades
Most gear and shaft forging work in Pakistan's automotive supply chain, done by PAAPAM-member shops along Lahore's GT Road and in Karachi's SITE and Bin Qasim areas, comes down to one branching question: does the part need a carburized wear surface, or does it need to be strong and hard through its whole cross-section? The answer sorts most jobs into one of three grades.
| Property | 8620 Alloy Steel | 4140 Alloy Steel | 5140 Alloy Steel |
|---|---|---|---|
| Category | Alloy Steel | Alloy Steel | Alloy Steel |
| Carbon (C) | 0.18 to 0.23% | 0.38 to 0.43% | 0.38 to 0.43% |
| Chromium (Cr) | 0.4 to 0.6% | 0.8 to 1.1% | 0.7 to 0.9% |
| Nickel (Ni) | 0.4 to 0.7% | Not published | Not published |
| Molybdenum (Mo) | 0.15 to 0.25% | 0.15 to 0.25% | Not published |
| Hardness | 149 HB | Not published | 167 HB |
| Density | 7.85 g/cm³ | 7.85 g/cm³ | Not published |
| Group | Nickel-chromium-molybdenum case-hardening steel | Chromium-molybdenum low alloy steel | Chromium alloy steel |
Confirm against the mill test certificate before specifying. Chat with us if you need help matching a grade to a job.
8620: when carburizing is part of the plan
8620 is deliberately low in carbon, 0.18 to 0.23%, because it's meant to be carburized, a hard case over a tough core. This is the standard grade for gears specifically, where tooth surfaces need wear resistance but the core needs to absorb shock without cracking. In the annealed condition it delivers 530 MPa tensile strength and 149 Brinell hardness, describing the core, the carburized surface reaches considerably higher hardness through the case-hardening process itself. Several forging shops in this trade already run carburizing in-house, worth confirming before quoting to make sure the customer has, or can access, that capability rather than needing to source it separately.
4140: the general-purpose middle ground
4140 runs higher carbon, 0.38 to 0.43%, and through-hardens rather than needing a carburized case. It's the default for shafts, general gears not requiring a case-hardened surface, and a wide range of forged machinery components. Its actual strength after heat treatment depends heavily on the temper applied, which is part of its appeal to a forge shop serving multiple customers, the same base composition can be tempered toward different final strength targets depending on the job. If a customer asks for "alloy steel" without more specification, 4140 is usually the safe first quote.
5140: for moderate strength without full alloy cost
5140 sits below 4140 in alloying, chromium only (0.700 to 0.900%), no molybdenum, giving it moderate hardenability and strength, 570 MPa tensile strength and 167 Brinell hardness in the annealed condition, at a lower material cost. It's the right call when a shaft or fastener needs more than plain carbon steel but doesn't need 4140's full alloy package, a genuinely useful middle option to keep in stock rather than defaulting every mid-strength job to the more expensive grade.
A quoting shortcut
Carburized gear, tooth surface wear resistance matters most: 8620. General shaft or gear, through-hardened, no case needed, mid-to-high strength: 4140. Moderate strength shaft or fastener, cost matters, full alloy hardenability isn't required: 5140. Getting this right on the first quote, rather than after a part fails in service, is what keeps a forging shop coming back to the same steel supplier, and it's a small enough set of questions that a supplier can walk through it with a customer in a two-minute phone call.
Worth remembering for repeat customers
Forging shops in this trade often order the same handful of grades repeatedly across different jobs for different end customers, so once a supplier understands which grade a given shop reaches for and why, quoting future orders gets faster for both sides, and it's easier to flag when a customer's request doesn't quite match what they've ordered before, sometimes a sign the specification changed and is worth a quick confirmation call.
Quoting a gear or shaft job? Send the application and load requirements and we'll help confirm the grade.